1. Field of the Invention
This invention is related to inductive power transfer, and more specifically to inductive power transfer using a relay coil.
2. Description of the Related Art
Wireless (i.e., inductive) power transfer has drawn renewed interest recently, particularly in the wireless charging of portable electronics products. The formation of the wireless power consortium which promotes the wireless power standard Qi has sped up the research and development of wireless power transfer technology. However, in the Qi standard version 1.1, the distance or gap between the surface of a wireless charging pad (or literally the transmitter coil) and the receiver coil of the load is typically limited to 5 mm. As more varieties of wireless charging systems will certainly appear, it is envisaged that some new wireless charging systems may benefit from a larger or extended gap between the transmitter coil and the receiver coil. For example, the transmitter coil may be installed under an office desk or a coffee/kitchen table while the portable electronic devices will be placed on the top surface of the desk or table, so the thickness of the desk or table will be included in the gap between the transmitter and receiver coils.
For wireless power transfer with an extended transmission distance, the conventional 2-coil system (i.e., one transmitter coil and one receiver coil separated by the transmission gap) loses efficiency quickly as the gap increases. Some systems are based on the maximum power transfer principle (i.e., based on impedance matching). However, it has been mathematically proven that, although the maximum power transfer principle can extend the transmission distance, it does so at the expense of energy efficiency. Any wireless power system based on impedance matching or the maximum power transfer theorem cannot achieve energy efficiency higher than 50%. Other systems are based on the maximum energy efficiency principle. The maximum energy efficiency principle is a better approach than the maximum power transfer principle because the overall energy efficiency can exceed 50%. However, as previously mentioned, even if the maximum energy efficiency approach is adopted in a conventional 2-coil system, the energy efficiency will drop exponentially with transmission distance. The use of systems with three or more coils has been proposed. However, there is an incomplete understanding of these types of systems.
Therefore, there is a need for improved approaches to wireless power transfer, especially for approaches that can efficiently transfer power across an extended transmission distance.